And if we break those 'vote neither down':
Newly reg: Biden 47, Trump 36
Previously reg: Trump 41, Biden 40

And one last cut (again, pooling all states / data)
All RVs: Biden 46, Trump 42
Our results: Biden 47, Trump 43
Validated 2018 voters: Biden 48, Trump 42
Validated 2016 voters: Biden 46, Trump 43

And worth noting that I'm not at all adjusting for the size of these various states, so Montana is getting 2x the weight of Florida, for example

All that said, this actually does make me feel pretty ok about the prospect of high turnout. If, like us, you've already baked the '18/20 Dem primary vote into your turnout model, then I think the new voters from there are probably pretty even